1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to an electro static discharge (hereinafter referred to as ESD)-protecting circuit.
2. Discussion of the Related Art
A conventional ESD-protecting circuit will be described with reference to the accompanying drawings.
FIG. 1 is a cross-sectional view showing a structure of a conventional ESD-protecting circuit which includes a p-type well 12 having a predetermined depth beneath a surface of a semiconductor substrate 11, first and second impurity diffusion regions 13 and 14 of n-type spaced apart from each other beneath the top surface of the semiconductor substrate 11, and a third impurity diffusion region 15 of p-type spaced apart from the second impurity diffusion region 14 of n-type and formed beneath top surface of the p-type well 12.
An input port Vin is connected to the first impurity diffusion region 13 of n-type, an ground port GND is connected to the second impurity diffusion region 14 of n-type, and a bias voltage Vp catching a bias of the p-well 12 is connected to the third impurity diffusion region 15. Though not shown in the figure, the ground port GND connected to the second impurity diffusion region 14 is also connected to an inner circuit.
The operation of an ESD-protecting circuit having the aforementioned structure is achieved by a punch-through generated between the input port Vin and the ground port GND when static electricity is applied to the input port Vin. In case that a plus static electricity is applied to a conventional ESD-protecting circuit, a significant inverse bias is caught between the input port Vin and the p-well 12 and a forward bias flows between the p-well 12 and the ground port GND. Thus, the operation of the circuit is achieved by the punch-through between the input port Vin and the ground port GND.
On the contrary, in case a minus static electricity is applied to the conventional ESD-protecting circuit, forward currents flow between the input port Vin and the ground port GND, but a PN junction is not destroyed.
However, the conventional ESD-protecting circuit has problems. If a strong plus static electricity is applied to it, a PN junction is destroyed, thus the inner circuit can not be protected.